The cyclic nucleotide gated channel (CNGC) is the final component of the visual transduction cascade within the retina rod. CNGCs close in response to a decrease in the amount of 3',5'-cyclic guanosine monophosphate (cGMP). The activation of this channel by cyclic nucleotides has been examined by measuring the current activated in excised membrane patches following exposure to various cyclic nucleotides. A homology model based on the crystal structure of cyclic AMP receptor protein (CRP), a transcription factor in E. coli, has been used to further explore the ligand sidechain interactions in order to account for differences in the apparent affinities of different ligands. The purpose of this study is to examine the ligand binding using a soluble binding domain construct and to determine the high resolution structure of the CNGC binding domain. The CNGC binding domain will be expressed as a chimera with the related CRP. The soluble chimera will be expressed in E. coli and purified for structural studies and ligand binding measurements. The atomic structure of the binding domain will provide details of how the cyclic nucleotide is bound and provide a framework for further functional studies on the channel to elucidate the CNGC residues responsible for ligand binding. Finally, if the chimera retains CRP ability to bind DNA it will be used to determine a ligand's ability to enact conformational changes in the binding domain.